As demonstrated by the huge number of patents existing in this field, the use of different security elements making the forgery of documents difficult has been extended in recent years. Some of these elements are detectable by human beings, meanwhile other security elements which are incorporated into documents require the use of special tools for detection thereof. These tools include spectroscopic methods such as UV-VIS absorption spectroscopy, fluorescence emission spectroscopy, IR spectroscopy or Raman spectroscopy.
Thus, luminescence pigments or substances have been incorporated into various security documents for certifying the authenticity thereof, the detection or observation of which requires the use of an excitation light in a particular region of wavelengths (for example UV light). Nevertheless, the use of this type of luminescence pigments or substances has some drawbacks including the limited amount of optical transitions (absorptions and emissions) with suitable properties for this application.
Raman spectroscopy in turn has also been described as a suitable method for detecting the authentication of documents. Raman Effect is based on an inelastic scattering of photon produced after impacting light on a material. In other words, an energy transfer between light and the material is produced such that the light coming out from the material has a frequency (or a wavelength or energy) different from that of the incident light. To enable observing this effect, it is necessary to use a strongly monochromatic light generally a laser radiation. The new frequency coming out with the light is directly related to the vibration frequencies of the bonds between the atoms forming the material, and therefore with the typical phonons of the network in the case of a crystal or a glass. Therefore Raman Effect, like infrared spectroscopy, is a vibration effect and in both cases the typical vibrations of the material, either the chemical bonds or the crystal network thereof are measured. This makes Raman effect a powerful tool for determining the structure and/or composition of materials. Raman spectroscopy can be used for example for recognizing drugs or for studying pigments in ancient work of arts, etc, and is very used in chemical and pharmaceutical industry. Nevertheless, not all materials present Raman Effect. Particularly, metals and some materials the crystal structure of which is cubic show no signal. However, the rest of the crystal structures, glasses and even gases and liquids present Raman Effect.
Since the Raman spectrum of each material is unique, different compounds have been incorporated in security elements as markers allowing the authentication thereof. Thus, the use of polydiacetylenes, for example, as active Raman compounds in security inks is described in U.S. Pat. No. 5,324,567. In this document these compounds in the form of particles with a maximum dimension of 40 microns are used. U.S. Pat. No. 5,718,754 in turn describes a pigment which has adsorbed on its surface a compound presenting azo, azomethine, or polycyclic chromophore groups showing Raman spectrum. Other compounds of organic origin used in the form of microparticles as Raman markers are described in US 2002/0025490.
Nevertheless, the use of these compounds as Raman markers does not involve an especially safe authentication system since the proper disclosure of the structure thereof would allow reproducing it, making the system readily forgeable provided that means suitable for the synthesis thereof is provided. Furthermore, when a single Raman material is selected for making an invisible image or a barcode, for example, areas in which there is a high concentration of a material (the active Raman material) which are not in the rest of the document which allows deducing the presence of an image are found and it can further allow knowing the material used using to that end suitable microanalysis techniques.
On the other hand, it is known in the state of the art that materials in the form nanoparticles have properties different from those that are shown in large material (in larger sizes greater than one micron). Thus, the position of the Raman peaks varying with the size of the nanoparticles has been described. In other words, nanoparticles of a particular material with a well defined particle size present a Raman spectrum with well defined peaks in well established positions. This same material but with a different particle size presents a spectrum very similar to the above but shifted in frequencies such that it is easy to distinguish both materials.
Document WO 2010/135351 describes nanoparticles comprising a core formed by an active marker and a metal coat. These nanoparticles can act as optical labels being useful for identifying or quantifying substances or objects and are applied as security measure for preventing forgery of documents, serialization, traceability, etc. Materials used as markers include conjugated polyaromatic compounds, porphyrins, phthalocyanines, metal oxides and ionic liquids. Documents US 2007/165209 and US 2006/038979 refer to method for providing a security element for documents particularly bills comprising applying a label to a portion of the document where said label comprises a metal nanoparticle, a Raman associated active molecule on the surface of the nanoparticle and a encapsulant surrounding said nanoparticle. The mentioned label can also be applied as an ink.
In all these cases, metal element is used as an amplifier of Raman signal without any effect on the spectrum itself therefore they do not involve an improvement as a security system, with the exception of the fact that they allow reducing the amount of active material and thereby increasing the difficulty for duplicating them.
Therefore, there is a clear need to develop new compositions and methods which make forging security documents difficult.